In carrying out voice communication by means of a conventional telephone network, the telephone set at the premise of each subscriber is linked by a twisted pair cable to the central office of the network. This office acts to interconnect all subscribers to the network so that they can communicate with each other via twisted pair cables.
In this modern age of information, a typical telephone subscriber has available at his residence or office not only a telephone set but also a fax machine, a desktop computer and other electronic devices capable of transmitting and receiving digital data to and from other telephone subscribers in the network as well as to Internet websites and other sites capable of processing digital data.
In order to be able to transform an existing telephone network which conveys voice signals over twisted pair lines into a multi-media network it is known to provide for this purpose as asymmetric digital subscriber line (ADSL). An ADSL is a point-to-point connected circuit which affords each subscriber to the network with a high-speed communication link that in addition to the usual telephone services affords many other services such as video-on-demand, conference video phone communication and high-definition TV as well as a full range of Internet services.
When an ADSL is associated with a telephone network, it then becomes necessary to place a modem at either end of the existing twisted pair telephone line conveying the voice signals. This modem serves to establish the following three channels of information:
Channel A: a high-seed downstream channel running from the central office of the network to an end user.
Channel B: a medium-speed upstream channel running from the end user to the central office.
Channel C: a conventional voice channel, commonly referred to as POTS, meaning Plain Old, Telephone Service.
In this arrangement, voice channel A is segregated from data channels A and B by bandpass filters to assure uninterrupted voice telephone service even in the event of a circuit failure in the ADSL system.
Currently available are two ADSL systems which comply with current regulatory telephone standards, namely: a split-type ADSL and a splitterless type. In an ADSL of the split-type, also known as a Full-Rate type, the voice signals which are produced concurrently with the digital data signals are split therefrom and conveyed to the central station of the network over a twisted pair cable, whereas the data signals are conveyed over another twisted pair cable.
This split is effected by a POTS splitter placed at each end of the telephone line, the splitter being provided with a low bandpass filter to permit only voice signals to be supplied to one of the twisted pair lines, and a high bandpass filter to permit only digital line signals to be fed to the other twisted pair cable.
An ADSL of the split type cannot be installed in residences or offices having the usual two-wire internal telephone line wiring. It is necessary in order to install a split-type ADSL in these facilities, to provide an additional two wire line running from the modem to the splitter. Such rewiring substantially raises the expenses incurred in installing an ADSL system.
In a multiple apartment building the incoming telephone lines go into a central box in the building, from which box extend telephone lines leading to each apartment. The cost of rewiring this building to accommodate a split-type ADSL system could be considerable and possibly prohibitive.
Existing ADSL systems of the “splitterless” type which function to convey both digital data and voice signals simultaneously over a single twisted pair telephone cable include either a POTS splitter to separate the voice from the data or a POTS Line Card for this purpose. These are relatively expensive components.
In order to obviate the need to include a POTS splitter in the digital signal processor of ADSL, U.S. Pat. No. 5,889,856 discloses an Integrated ADSL Line Card capable of processing both data and voice signals on the same board. But while this ADSL Line Card reduces the total cost of the unit, the card is difficult to apply, for it must transmit a composite signal containing both the high-frequency digital data and low-frequency voice signals by way of the same driver circuits.
Another limitation of existing ADSL splitterless systems is that they include an interleaving device which introduces substantial delays in the transmission of digital data.
However the greatest practical drawback of existing splitter ADSL system is that it gives the telephone subscriber only a single baseband channel for voice communication. In many cases, this signal channel falls short of satisfying the subscriber's voice communication requirements.